In general, cemented carbide (a WC-Co alloy or an alloy prepared by adding a carbonitride such as Ti (titanium), Ta (tantalum) or Nb (niobium) thereto) has been employed as a tool for cutting. Following the recent speed-up of cutting, however, the ratio of usage of a hard alloy tool comprising a substrate of cemented carbide, cermet, a cubic boron nitride sintered body or alumina-based or silicon nitride-based ceramics whose surface is coated with at least one coating layer of a compound formed by at least one first element selected from the group IVa elements, the group Va elements and the group Via elements of the element periodic table, Al (aluminum), Si and B and at least one second element selected from B, C, N and O (if the first element is only B, the second element is other than B) by CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition) so that the thickness of the coating layer is 3 to 20 μm is increased.
This cutting tool has a rake face coming into contact with chips of a workpiece and a flank face coming into contact with the workpiece itself in cutting, and a portion corresponding to a ridge where the rake face and the flank face intersect with each other (and a portion around the same) is referred to as an insert ridge portion.
In recent years, the cutting speed has been more increased in order to further improve cutting efficiency, and higher wear resistance has been required to the cutting tool following this. However, toughness is reduced if high wear resistance is required, and hence compatibility between high wear resistance and high toughness is required.
As an attempt for satisfying such requirement, Japanese Patent Laying-Open No. 05-177411 (Patent Document 1), for example, pays attention to residual tensile stress of a cover layer caused when the cover layer is formed on a substrate by chemical vapor deposition (CVD) at a high temperature and thereafter cooled to the room temperature, assumes that this tensile stress brings reduction of toughness of a cutting tool, and proposes a solution thereto. In other words, this tensile stress results from the difference between the thermal expansion coefficients of the substrate and the cover layer, and a technique of improving toughness (fracture resistance) while maintaining high wear resistance by first forming a first cover layer having such tensile stress on the substrate, forming prescribed cracks on this first cover layer and thereafter forming a second cover layer having compressive stress on the first cover layer is employed.
Japanese Patent Laying-Open No. 05-177412 (Patent Document 2), paying attention to tensile stress of a cover layer similarly to the above, employs an approach different from the above and proposes a structure prepared by forming an inner cover layer having tensile stress on a hard ceramics substrate and forming an outer cover layer having compressive stress thereon. Further, Japanese Patent Laying-Open No. 05-177413 (Patent Document 3) proposes a cutting tool, comprising a substrate of cermet, having a structure similar to that of Patent Document 2.
On the other hand, Japanese Patent Laying-Open No. 06-055311 (Patent Document 4) proposes a cutting tool prepared by forming a hard cover layer on a substrate of cemented carbide by chemical vapor deposition while substantially eliminating tensile stress from the hard cover layer on a rake face portion while holding tensile stress of the hard cover layer on a flank face portion.
Japanese Patent No. 3087465 (Japanese Patent Laying-Open No. 06-079502, Patent Document 5) proposes a cutting tool prepared by forming a hard cover layer having compressive stress distribution substantially uniform over the entire insert on the surface of a titanium carbonitride-based cermet substrate and increasing compressive stress of a rake face portion beyond compressive stress of a flank face portion by at least 49 MPa by performing shot blasting on the hard cover layer.
However, while compatibility between toughness and wear resistance can be attained to a certain extent in each of the aforementioned proposals, higher performance is required to a cutting tool under the present circumstances of the cutting tool, and development of a cutting tool sufficiently satisfying such performance is demanded.
Patent Document 1: Japanese Patent Laying-Open No. 05-177411
Patent Document 2: Japanese Patent Laying-Open No. 05-177412
Patent Document 3: Japanese Patent Laying-Open No. 05-177413
Patent Document 4: Japanese Patent Laying-Open No. 06-055311
Patent Document 5: Japanese Patent No. 3087465 (Japanese Patent Laying-Open No. 06-079502)